A great variety of transducers exist in the prior art which are used to measure force and pressure in different environments. Many of these sensors use piezoresistive, piezoelectric, and capacitive methods for measuring force and pressure. All the above transducers produce relatively low level analogue signals wherein both the zero pressure reading and the scale factor can vary as a function of temperature to one degree or another. Moreover, to get a digital signal from such a device requires a conversion of the analogue signal to a varying frequency. The consequences of the above are less precise force or pressure transducers.
On the other hand, transducers which rely on vibrating structures for the measurement of physical properties such as pressure and force have a number of advantages over the aforesaid conventional analogue transducer constructions. Although the prior art is replete with a myriad of vibrating structure transducers, all operate on the same basic principle; exciting a structure to its resonant frequency by external electrical means and applying a pressure or force to change the resonant frequency. The resulting change of the resonant frequency is proportional to the applied external stress and hence, is a measure of force or pressure. Moreover since the output is a change in frequency, digital data is obtained directly.
Older devices such as vibrating-cylinder pressure transducers manufactured by Hamilton-Standard and others, utilize the magneto-strictive effect to cause mechanical movement in the sensor, and the movement of the cylinder is detected by magnetic resonances.
There is also a wide body of prior art concerning resonant diaphragm pressure transducers using either metal or silicon diaphragms. In these devices, a diaphragm is excited into resonance and the change of resonant frequency with applied stress to the diaphragm is measured.
More recently, silicon resonant beam transducers have been disclosed. In this class of devices, a beam is mechanically coupled to a diaphragm in such a way that application of pressure to the diaphragm will induce a change in the state of stress of the beam causing its resonant frequency to vary in accordance with the applied pressure. The beam can be excited into mechanical vibration by applying an electrostatic force between the beam and the underlying diaphragm. The deflection of the beam is detected by means of piezoresistive sensors on one end of the beam. Such a resonant beam transducer and its method of fabrication were described in an article by Kurt Peterson et al. entitled, "Resonant Beam Pressure Sensor Fabricated with Silicon Fusion Bonding," published by the IEEE in 1991.
The present invention takes cognizance of the prior art but offers many fundamental improvements. All previous resonating transducer structures made use of a single resonating member whose resonant frequency was changed by the applied mechanical input. While this approach yields advantages over an analogue approach, it does not yield the optimum characteristics. However, in the present invention, two resonating members are utilized each of which may be affected by an externally applied stress to a different extent. For instance, one resonator may be totally unaffected by applied pressure while the other resonator's natural frequency will depend on the applied pressure. If now the two resonant frequencies are inputed into suitable electronics the difference or beat frequency between the two resonators may be obtained.
In this way since each beam's resonant frequency will change slightly with respect to temperature, this difference, however, will cancel out. Moreover, by measuring the beat or difference frequency, a greater inherent accuracy will result. If for instance, one can resolve the frequency to one part in 10.sup.5 and each natural frequency is of order 100 kHz and the difference is 10 kHz, an accuracy enhancement of a factor of 10 will result.
In addition, as will be shown in the following detailed description of the invention, the new improved method of fabrication utilizing diffusion-aided fusion bonding as described in U.S. Pat. No. 5,286,671 entitled FUSION BODING TECHNIQUE FOR USE IN FABRICATING SEMICONDUCTOR DEVICES issued to Kurtz et al. on Feb. 15, 1994 and assigned to Kulite Semiconductor Products, Inc., the assignee herein, which application is expressly incorporated herein by reference in its entirety, will enable the device to be operated at significantly higher temperatures than any other prior art silicon resonating structure. In the prior art, the piezoresistive sensors which act to measure the frequency response of the beams, have been made using p-n junction isolation techniques, whether the piezoresistive sensors were diffused or ion-implanted. In the present invention, making use of the methods taught in U.S. Pat. No. 5,286,671, the sensor network can be dielectrically isolated from the resonant beam. This not only makes for a cheaper, smaller structure but insures that the device can be used at temperature far in excess of the silicon p-n junction breakdown. The same is also true with respect to the voltage used to provide the electrostatic force between the beam and the diaphragm since, the beam can be dielectrically isolated from the diaphragm.
Many other unanticipated advantages arise from the proposed structure. For instance, if it is desired to measure an absolute pressure, i.e., that above vacuum, it is sufficient to insure that the back sides of each resonating beam structure is exposed to vacuum and only one of the resonating beams is connecting to a complaint diaphragm. In this way the application of an absolute pressure to the front side of the structure will only result in the change of frequency of the stress coupled beam. Thus, the difference frequency will be a direct measure of absolute pressure.
In a similar manner, if one wants to measure differential or gage pressure, one needs only to make both diaphragms compliant and expose one diaphragm to atmospheric pressure and the other to the gage pressure to be measured. The difference in resonant frequency between the two diaphragms pressure, coupled beams will give a response proportional to the gage or differential pressure.